BOX filter missed some edge-case pixels

What did you do?

I down-sample an image with BOX filter.

Here is the image:

table-51.png (width="2755" height="1837")

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What did you expect to happen?

All pixels are counted while down-sampling.

The result should look like this (688x459, 459x306):

box-js-v3-table-51.png-688x459-49c214b-pil-like.png

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box-js-v3-table-51.png-459x306-20d1f1f-pil-like.png

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What actually happened?

Some edge-case pixels were skipped.

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Downscaled 4 times to 688x459:

box-pil-table-51.png-688x459-49c7ade.png

missed a central vertical line.

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Downscaled 6 times to 459x306:

box-pil-table-51.png-459x306-20d5e62.png

missed a central horizontal line.

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What are your OS, Python and Pillow versions?

• OS: Win 10
• Python: 3.12
• Pillow: 12.3.0

file_path = "./table-51.png"
scale = 4  # scale(-down)

image = Image.open(file_path)
image_gray = image.convert("L")
wh = (int(image.width / scale), int(image.height / scale))
image = image_gray.resize(wh, Image.Resampling.BOX)

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Search words: off-by-one error, range rounding.

Start with the provided Image.resize example using Image.Resampling.BOX and the linked table-51.png input. Compare the 688x459 and 459x306 results with the expected images, focusing on the skipped central vertical and horizontal lines. Done means BOX downsampling counts all pixels and produces the expected edge-case output.